The invention relates to films used to make pouches filled with flowable materials, e.g. liquids.
As used herein, the term xe2x80x9cflowable materialxe2x80x9d does not include gaseous materials, but encompasses materials which are flowable under gravity or may be pumped. Such materials include liquids, e.g. milk, water, fruit juice, oil; emulsions, e.g. ice cream mix, soft margarine; pastes, e.g. meat pastes, peanut butter; preserves, e.g. jams, pie fillings, marmalade; jellies; doughs; ground meat, e.g. sausage meat; powders, e.g. gelatin powders, detergents; granular solids, e.g. nuts, sugar; and like materials. The invention described herein is particularly useful for flowable foods, e.g. milk.
It is well known to package flowable materials, for example, milk, on a pouch-forming apparatus such as a so-called vertical form, fill and seal apparatus. Typically, using such an apparatus, a flat web of synthetic thermoplastic film is unwound from a roll and formed into a continuous tube in a tube-forming section, by sealing the longitudinal edges on the film together to form a so-called lap seal or a so-called fin seal. The tube thus formed is pulled vertically downwards to a filling station. The tube is then collapsed across a transverse cross-section of the tube, the position of such cross-section being at a sealing device below the filling station. A transverse heat seal is made, by the sealing device, at the collapsed portion of the tube, thus making an air-tight seal across the tube. The material being packaged enters the tube above the transverse heat seal in a continuous or intermittent manner, thereby filling the tube upwardly from the transverse heat seal. The tube is then allowed to drop a predetermined distance usually under the influence of the weight of the material in the tube. The jaws of the sealing device are closed again, thus collapsing the tube at a second transverse section, which may be at, above or below the air/material interface in the tube, depending on the nature of the material being packaged and the mode of operation of the process. The sealing device seals and severs the tube transversely at the second transverse section. The material-filled portion of the tube is now in the form of a pillow shaped pouch. Thus, the sealing device has sealed the top of the filled pouch, sealed the bottom of the next-to-be formed pouch and separated the filled pouch from the next-to-be-formed pouch, all in one operation. Variations on pouch-forming machines and in particular on this type of vertical form, fill and seal apparatus are either known or conceivable. For example, the forming and sealing functions may be performed separately from severing function on separate machines. Also, the jaws of the sealing device could move to the next sealing position rather than have the film drop to the next position or there could be two sets of sealing jaws that seal both transverse ends simultaneously. Further, instead of forming a tube, two pieces of film could be fed into the machine and the pouch could be made by four seals, two longitudinal and two transverse.
Examples of the vertical, form, fill and seal apparatus of the type described above are the Prepac(copyright) IS-6 or IS-7 liquid packaging machine.
A sealing device commonly used is a so-called impulse sealer which has a sealing element mounted in sealing jaws and electrically insulated therefrom. In operation, the sealing jaws are closed and an electrical current is caused to flow through a sealing element, e.g. a wire, for a fraction of the time that the jaws are closed. The jaws remain closed during a cooling or equilibration period, during which the seal forms, before the sealing jaws are opened. The synthetic thermoplastic film must be able to provide a transverse seal that supports the weight of the flowable material, e.g. liquid, in the next-to-be-formed pouch. It is important that the cooling of the seal takes effect before the weight of the liquid can weaken or rupture the bottom seal. Sealing may also be carried out using any other suitable heat-sealing apparatus.
In the operation of pouch-forming machines, it is known to use film made from a blend containing 70-90% linear ethylene/butene-1 copolymer having a density of about 0.919 g/cm3 and a melt index of about 0.75 dg/min and 10-30% high pressure polyethylene, i.e. a homopolymer, having a melt index of about 2 to 10 dg/min and a density of from 0.916 to 0.924 g/cm3. Density is determined by ASTM Procedure D1505-85 and melt index by ASTM Procedure D1238-90B (Condition E). Such films having a thickness of about 76 micrometers, have been used commercially on vertical form, fill and seal apparatuses for making pouches containing about 1.3 liters of milk. Pouches made from such film tend to suffer from defective seals, i.e. a tendency to have weak transverse end/and longitudinal seals even though the operating conditions of the impulse sealer have been optimized. Defective seals may lead to the phenomenon known as xe2x80x9cleakersxe2x80x9d, in which the flowable material, e.g. milk, may escape from the pouch through pinholes that develop at or close to the seal. It has been estimated that leakers account for about 1-2% of the 1.3 liter milk pouch production.
U.S. Pat. No. 4,521,437 of W. J. Storms, issued Jan. 4, 1985, discloses the use of pouches of ethylene/octene-1 copolymer film in the packaging of flowable materials. That patent discloses that pouches made from the ethylene/octene-1 copolymer provide superior leaker performance, i.e. a lower percentage of leakers, compared with related films described above that are formed from linear ethylene/butene-1 copolymers.
It has now been surprisingly found that further improvements in leaker performance may be achieved using ethylene copolymers manufactured using a single-site polymerization catalyst.
Accordingly, the present invention provides a film for making pouches containing a flowable material, said pouch having at least two heat sealed ends, said film being made from a composition comprising a copolymer of ethylene and at least one C4-C10 alpha-olefin manufactured in a polymerization process using a single-site polymerization catalyst.
The present invention further provides a pouch containing a flowable material, said pouch having at least two heat sealed edges, wherein the film is made from a composition comprising a copolymer of ethylene and at least one C4-C10 alpha-olefin manufactured in a polymerization process using a single-site polymerization catalyst.
The present invention still further provides a process for making pouches filled with a flowable material, using a vertical form, fill and seal apparatus, in which process each pouch is made from a flat web of film by forming a tubular film therefrom with a longitudinal seal and subsequently flattening the tubular film at a first position and transversely heat sealing said tubular film at the flattened position, filling the tubular film with a predetermined quantity of flowable material above said first position, flattening the tubular film above the predetermined quantity of flowable material at a second position and transversely heat sealing said tubular film at the second position, the improvement comprising making the pouches from a flat web of film made from a composition comprising a copolymer of ethylene and at least one C4-C10 alpha-olefin manufactured in a polymerization process using a single-site catalyst.
Most particularly, the present invention provides a process for making pouches filled with a flowable material, using a vertical form, fill and seal apparatus, in which process each pouch is made from a flat web of film by forming a tubular film therefrom with a longitudinal seal and subsequently flattening the tubular film at a first position and transversely heat sealing said tubular film at the flattened position, continuously filling the tubular film with flowable material above said first position, flattening the tubular film above a predetermined quantity of flowable material at a second position and transversely heat sealing said tubular film at the second position, the improvement comprising making the pouch from a film comprising at least one layer formed from a blown film formed from a blend comprising a linear polymer of ethylene with at least one C4-C10 alpha-olefin manufactured in a single-site catalyst polymerization process, and at least one of the following
a) a linear polymer of ethylene with at least one C4-C10 alpha-olefin made by a multi-site catalyst polymerization process;
b) a high pressure low density polyethylene; and
c) additives selected from the group consisting of stabilizers, antiblock additives and extrusion aids;
and, the vertical form, fill and seal apparatus includes impulse sealing means for making transverse seals in the film through the flowable material.